Trimethylolpropane is a trihydric alcohol which is of significance for the production of coating materials, polyurethanes and polyesters, for example of alkyd resins. Trimethylolpropane is produced industrially by condensation reaction of n-butyraldehyde with formaldehyde according to different variants.
In what is called the hydrogenation process, at least two moles of formaldehyde are added onto one mole of n-butyraldehyde in the presence of a catalytic amount of a tertiary amine via the monomethylolbutyraldehyde intermediate to initially give dimethylolbutyraldehyde, which is then converted to trimethylolpropane in a hydrogenation step. According to the process described in WO98/28253 A1, formaldehyde is used with an up to eight-fold molar excess. The reaction mixture obtained from the aldol addition step is worked up either by distillation or by phase separation. In the distillative workup, unconverted or partly converted starting compounds are drawn off as volatile components and recycled into the reaction stage, while the bottom product is converted further. If, instead of the distillative workup, the reaction mixture is separated in a phase separator into the aqueous and organic phases, the organic phase is returned to the aldol addition and the aqueous phase is processed further. There follows a catalytic and/or thermal treatment in order to convert monomethylolbutyraldehyde to dimethylolbutyraldehyde. By-products formed are removed by distillation and the bottom product of this distillation is subsequently catalytically hydrogenated to obtain trimethylolpropane. The crude trimethylolpropane obtained is subsequently subjected to a purifying distillation. After removal of low and medium boilers, purified trimethylolpropane is obtained as an intermediate fraction, while higher-boiling condensation products within which trimethylolpropane equivalents are bound are obtained as the tailings or bottoms fraction.
In addition to the hydrogenation process, trimethylolpropane is also prepared industrially by what is known as the Cannizzaro reaction. In a first reaction stage, n-butyraldehyde and formaldehyde are reacted with addition of stoichiometric amounts of a base to give dimethylolbutyraldehyde, which is subsequently reduced with excess formaldehyde to give trimethylolpropane, while one equivalent of formate is formed simultaneously. Typically, the base used is an aqueous solution of an alkali metal or alkaline earth metal compound, for example sodium hydroxide, potassium hydroxide or calcium hydroxide. Since one equivalent of alkali metal or alkaline earth metal formate is obtained as a coproduct in the Cannizzaro process, the economic viability of this process variant also depends on the marketing opportunities for this coproduct. The workup of the aqueous reaction solution obtained, which comprises trimethylolpropane, akali metal or alkaline earth metal formate and excess base, is effected generally by extraction. After neutralization of the excess base, the aqueous solution is extracted with an organic solvent, for example with ethyl acetate. The organic phase is separated from the aqueous phase, which comprises the alkali metal or alkaline earth metal formates in dissolved form, and, after removal of the extractant, trimethylolpropane is obtained by distillation. The resulting trimethylolpropane can be subjected to further purification processes. According to U.S. Pat. No. 5,603,835, an aqueous solution is first prepared from resulting trimethylolpropane, and is extracted once again with an organic solvent, for example with methyl tert-butyl ether. Trimethylolpropane is obtained from the resulting aqueous solution with an improved colour number of less than 100 APHA units.
According to the process known from U.S. Pat. No. 5,948,943, the aqueous, crude reaction solution obtained after the Cannizzaro reaction is treated with a suitable organic solvent at such a temperature that only one liquid phase leaves the extraction vessel. In the subsequent cooling outside the extraction vessel, the aqueous phase separates from the organic phase, and trimethylolpropane can be isolated from the aqueous phase with a colour number of less than 100 APHA.
It is likewise known that the Cannizzaro reaction can be performed with an organic base, for example with a tertiary amine. According to the procedure known from WO97/17313 A1, formaldehyde is prepared with n-butyraldehyde in the presence of stoichiometric amounts of a tertiary amine, forming one equivalent of ammonium formate. Subsequently, water, excess tertiary amine and excess formaldehyde are removed from the crude mixture, and the remaining mixture is heated. This dissociates the ammonium formates to the tertiary amine and formic acid, and the tertiary amine and further volatile constituents are removed, resulting in the formation of trimethylolpropane formate. The tertiary amine removed is either recycled into the Cannizzaro stage or used as a catalyst for the transesterification of the trimethylolpropane formate in a downstream reaction with an added lower aliphatic alcohol. The trimethylolpropane released is subsequently isolated from the crude product.
Irrespective of whether the preparation of trimethylolpropane is effected by the hydrogenation process using catalytic amounts of a tertiary amine, by the Cannizzaro process with molar amounts of a tertiary amine and subsequent transesterification of the trimethylolpropane formate formed, or by the Cannizzaro process with molar amounts of alkali metal or alkaline earth metal hydroxides and the extractive removal thereof, the crude trimethylolpropane obtained is subjected to a single or multiple distillative purification, which is effected under reduced pressure due to the high boiling point. According to DE 100 58 303 A1, the distillative workup of the trimethylolpropane is combined with an ion exchanger treatment, in which case either the aldolization output or the hydrogenation output is contacted with a strongly basic ion exchanger before the distillative workup.
DE 1 768 348 B discloses reaction of two different aldehydes, for example acetaldehyde and butyraldehyde, with formaldehyde in an aqueous alkaline medium. The reaction mixture obtained is first neutralized by adding acid, freed of suspended solids and then treated with acidic and basic ion exchangers.
Distillative purification of crude trimethylolpropane gives rise not only to high boilers and residues but also to fractions having a lower boiling point compared to trimethylolpropane. These forerun fractions comprise, as well as low boilers, for example water, methanol or solvents, also certain amounts of trimethylolpropane itself, according to the separation sharpness in the distillative workup. Additionally present in the forerun fraction are derivatives of trimethylolpropane which have formed by preceding reactions with formaldehyde and methanol and have a boiling point which is lower than or comparable to trimethylolpropane.
Among these derivatives, particularly formaldehyde-containing acetals are represented, which are characterized by the structural element —O—CH2—O— and can also be regarded as formals. Among the formals, the following linear and cyclic formals of trimethylolpropane can be described structurally:
Monocyclic formal of trimethylolpropane:

Methyl (monolinear) formal of trimethylolpropane:C2H5C(CH2OH)2CH2OCH2OCH3   Formula II
Methyl (bislinear) formal of trimethylolpropane:C2H5C(CH2OH)2CH2OCH2OCH2OCH3   Formula III
In this context, the monocyclic formal of trimethylolpropane (I) boils at a lower temperature than trimethylolpropane itself. The methanol-derived formals (II) and (III) have a boiling point comparable to trimethylolpropane.
In addition, 2-methylbutanol, 2,2-dimethylpropane-1,3-diol, 2-ethylpropane-1,3-diol and 2-ethyl-2-methylpropane-1,3-diol are present in the forerun fraction.
In the context of the workup of high-boiling fractions and residues obtained in the distillative workup of trimethylolpropane, a number of processes are proposed to dissociate especially formaldehyde-containing acetals and to release trimethylolpropane, in order in this way to improve the yield of the overall trimethylolpropane preparation process. According to WO 2004/013074 A1, the high-boiling fractions and distillation residues obtained in trimethylolpropane preparation are treated with acid, and the water content in the reaction mixture should be 20-90% by weight. It is possible either to obtain trimethylolpropane by distillation from the acid-treated product or to recycle the treated product into the hydrogenation stage of dimethylolbutyraldehyde to give trimethylolpropane. The hydrogenating dissociation of linear or cyclic acetals in aqueous solutions in the presence of a heterogeneous hydrogenation catalyst to give the desired polyhydric alcohol is known from DE 198 40 276 A1. The process requires hydrogenation temperatures above 160° C. in order to suppress the harmful influence of formates, which may still be present particularly in the case of working by the Cannizzaro process, on the hydrogenation performance of the catalyst. According to WO 97/01523 A1, the hydrogenation temperature can be lowered, but a high weight ratio of the catalytically active metal to the cyclic formal then has to be established in order to achieve an acceptable dissociation rate. Moreover, in WO 97/01523 A1 and DE 198 40 276 A1, all working examples are conducted with ruthenium on activated carbon catalysts in order to demonstrate the executability of the processes disclosed. For catalytic dissociation of the formaldehyde-containing acetals, the prior art discloses the use of expensive noble metal catalysts at elevated temperature or use thereof in a comparatively large amount, based on the formaldehyde-containing acetals. The hydrogenating catalytic dissociation can likewise be conducted in the presence of an acid, for example in the presence of a lower carboxylic acid or acidic solids.